When a certain compound forms two or more crystalline states, these different crystalline states are referred to as polymorphism. It is generally known that the stability varies with each polymorph (crystal form) of polymorphism. For example, Japanese Unexamined Patent Publication (KOKAI) No. 62-226980 describes that two polymorphs of prazosin hydrochloride each having a different stability, thus exerting an influence on the results of the long-term storage stability. Also, Japanese Unexamined Patent Publication (KOKAI) No. 64-71816 describes that a specific one among different polymorphs of buspirone hydrochloride is advantageous in view of retention of specific physical properties under storage or manufacturing conditions.
As described above, a specific polymorph is superior in stability, sometimes. Accordingly, in case where a plurality of polymorphs are present, it is important to develop a technique of preferentially producing each polymorph. Particularly, in case where a pharmaceutical composition comprising a useful compound as a drug is produced, it is suitable to control polymorphism so as to formulate a pharmaceutical composition containing only a superior specific polymorph.
As described in International Publication WO92/09279, it is known that 2-(3-cyano-4-isobutyloxyphenyl)-4-methyl-5-thiazolecarboxylic acid represented by the following formula has an activity for inhibiting xanthine oxidase. ##STR1##
However, the above-mentioned publication does not describe polymorphism, and therefore, the crystal form of 2-(3-cyano-4-isobutyloxyphenyl)-4-methyl-5-thiazolecarboxylic acid studied in the publication is not clear. It is only assumed from the experimental operation described therein that it is an ethanolate. The evaluation of the activity described in that publication is not conducted in a solid state and, therefore, there is not any description about characteristics of the polymorph.
Polymorphism is meaningless unless solid physical properties exert an influence on biological activity, physiochemical properties or an industrial manufacturing method of the substance. For example, when using as a solid preparation in animals, it is important that the presence or absence of polymorphism is confirmed in advance and a technique of selectively producing a desired polymorph is developed. In case where the substance is stored for a long period of time, a question is how the crystal form can be retained in a stable manner. It is also an important object to develop a technique of producing the crystal form in an industrially easy and reproducible manner.